Radio Telescopes Reveal Unseen Galactic Cannibalism

Radio-telescope images have revealed previously-unseen
galactic cannibalism -- a triggering event that leads to
feeding frenzies by gigantic black holes at the cores of
galaxies. Astronomers have long suspected that the extra-bright
cores of spiral galaxies called
Seyfert galaxies are powered by
supermassive black holes consuming material. However, they
could not see how the material is started on its journey
toward the black hole.

Visible-light (left) and radio (right) image of galaxy pair:
Radio image shows gas streaming between galaxies.CREDIT:Kuo et al., NRAO/AUI/NSFClick on image for more graphics.

One leading theory said that Seyfert galaxies have been
disturbed by close encounters with neighboring galaxies, thus
stirring up their gas and bringing more of it within
the gravitational reach of the black hole. However, when astronomers
looked at Seyferts with visible-light telescopes, only a small
fraction showed any evidence of such an encounter. Now, new
images of hydrogen gas in Seyferts made using the National
Science Foundation's
Very Large Array (VLA) radio telescope
show the majority of them are, in fact, disturbed by ongoing
encounters with neighbor galaxies.

"The VLA lifted the veil on what's really happening with these
galaxies," said Cheng-Yu Kuo, a graduate student at the University
of Virginia. "Looking at the gas in these galaxies clearly showed
that they are snacking on their neighbors. This is a dramatic
contrast with their appearance in visible starlight," he added.

The effect of the galactic encounters is to send gas and dust
toward the black hole and produce energy as the material
ultimately is consumed.
Black holes, concentrations of matter
so dense that not even light can escape their gravitational pull,
reside at the cores of many galaxies. Depending on how rapidly
the black hole is eating, the galaxy can show a wide
range of energetic activity. Seyfert galaxies have the mildest
version of this activity, while
quasars and
blazars are hundreds
of times more powerful.

The astronomers picked a number of relatively nearby Seyfert
galaxies that had previously been observed with visible-light
telescopes. They then carefully studied the Seyferts with the
VLA, specifically looking for radio waves emitted by hydrogen
atoms. The VLA images showed the vast majority of the Seyferts
were disturbed by encounters with neighbor galaxies.

By comparison, similar VLA images of inactive galaxies showed
that very few were disturbed. "This comparison clearly shows
a connection between close galactic encounters and the
black-hole-powered activity in the cores," said Ya-Wen Tang,
who began this work at the Institute of Astronomy & Astrophysics,
Academia Sinica (ASIAA), in Taiwan and now is a graduate student
at the National Taiwan University.

"This is the best evidence yet for the fueling of Seyfert
galaxies. Other mechanisms have been proposed, but they
have shown little if any difference between Seyferts and
inactive galaxies," Tang added.

"Our results show that images of the hydrogen gas are a
powerful tool for revealing otherwise-invisible gravitational
interactions among galaxies," said Jeremy Lim, also of ASIAA.
"This is a welcome advance in our understanding of
these objects, made possible by the best and most extensive
survey ever made of hydrogen in Seyferts," Lim said.

Kuo, Tang and Lim worked with Paul Ho, of ASIAA and the
Harvard-Smithsonian Center for Astrophysics. The scientists
reported their findings in the Astrophysical Journal.